1. Field of the Invention
The present invention relates to a magnetic recording medium such as a video tape for analog or digital recording and a data tape, particularly, relates to a magnetic recording medium, which can provide a higher reproduction output and a higher C/N (carrier to noise ratio) in a short wavelength recording range.
2. Description of the Related Art
Recently, it is required for a magnetic recording medium such as a video tape and a data tape to be excellent in recording and reproducing characteristics in a short wavelength recording range along with a recording and reproducing apparatus, which is advanced to higher recording density. In the case of a digital video tape (hereinafter referred to as DVC tape), for example, a so-called evaporation tape, which adopts a ferromagnetic metal thin film, has been put into practical use for the purpose of improving recording density.
Such an evaporation tape formed with a ferromagnetic metal thin film is manufactured by the vacuum evaporation method and has excellent recording and reproducing characteristics in a short wavelength recording range. With respect to a magnetic layer of evaporation tape, such a material as Co (cobalt), Co—Ni, Co—O, and Co—Ni—O has been examined. In the case of an evaporation tape for Hi-8 system video tape recorder (VTR) that is commonly available in the market, the evaporation tape is formed with a Co—Ni—O layer over a polymeric substrate.
Further, in the case of a DVC tape for digital video recording that is commonly available in the market, such a DVC tape is an evaporation tape formed with a Co—O magnetic layer.
FIGS. 6(a) and 6(b) are an enlarged cross sectional view of a conventional magnetic recording medium according to the prior art. In FIG. 6(a), an evaporation tape is composed of a polymeric substrate 1 and a magnetic layer 30 that is formed over the polymeric substrate 1. As shown in FIG. 6(a), the magnetic layer 30 is formed with a plurality of columns 30c that are composed of crystalline particles. Each column 30c tilts with respect to the normal line of the polymeric substrate 1. A forming method of such a magnetic layer is called an oblique evaporation method.
With referring to FIG. 11, a manufacturing method of conventional magnetic recording medium is explained next.
FIG. 11 is a plan view of a general manufacturing apparatus for manufacturing an evaporation tape by using the oblique evaporation method according to the prior art. In FIG. 11, the polymeric substrate 1 is pulled out from a supply roll 3 to an arrow “a” direction, and runs along the outer circumference of a cylindrical cooling can roll 4, and finally taken up by a take-up roll 5 with being toward an arrow “b” direction. An evaporation material 12 such as Co, which becomes crystalline particles and forms a magnetic layer, is filled in a crucible 11 and evaporates by being heated by irradiating an electron beam 13.
An atom evaporated from the evaporation material 12 (hereinafter referred to as evaporated atom) is reflected by a thin plate 16 or a reflector 16, which is disposed on a position that faces toward the polymeric substrate 1 approximately and heated more than the melting point of Co, and deposited on the surface of the polymeric substrate 1 that is running along the cylindrical cooling can roll 4. The magnetic layer having oblique magnetic anisotropy, which is formed by the evaporated atom reflected by the reflector 16, is formed on the surface of the polymeric substrate 1, and then continuously formed by evaporated atoms that fly directly from the crucible 11 and deposit on the surface of the polymeric substrate 1.
As mentioned above, the evaporated atom of the evaporation material 11 is deposited on the surface of the polymeric substrate 1 within an opening section between the reflector 16 and an anti-deposition plate 8. An ending point of deposition is almost an edge portion 8a of the anti-deposition plate 8 in the opening section side. By injecting oxygen gas through an oxygen gas injection spout 10 that is installed adjacent to the edge portion 8a, an oxide layer of the evaporation material 12 can be formed on the polymeric substrate 1.
With respect to a method of improving recording and reproducing characteristics of an evaporation tape, several methods such as providing discontinuous multi-layers of evaporation layer or controlling oxygen gas density in a magnetic layer have been introduced.
For example, the Japanese Patent Laid-open Publication No. 8-315346/1996 discloses a multi-layered evaporation tape shown in FIG. 6(b). As shown in FIG. 6(b), the multi-layered evaporation tape is composed of a first magnetic layer 31 and a second magnetic layer 32, wherein they are formed through an evaporation process twice and discontinuous with respect to each other.
The Japanese Patent Laid-open Publication No. 10-324965/1998 discloses an evaporation tape as a recording medium having a magnetic layer in which crystalline particles are disposed continuously. In the evaporation tape, columns that constitute the magnetic layer are oriented partially in a circular arc in the polymeric substrate side, and then oriented in columnar.
The Japanese Patent Laid-open Publication No. 2001-143235 discloses a magnetic recording medium that is composed of a magnetic layer having a columnar crystalline structure. Each column is divided into three sections. In each section, oxygen density, magnetic flux density or a tilt angle of each column is specified.
By the magnetic recording medium according to the prior arts mentioned above, there existed problems such that the magnetic recording medium does not have high coercive force, or hardly provides a high reproduction output or a high C/N in a short wavelength recording range.
Further, in the case of the multi-layered evaporation tape that is disclosed in the Japanese Patent Laid-open Publication No. 8-315346/1996, there existed a problem such that productivity is deteriorated because 2-time evaporation process is essential.